1. Field of the Invention
The present invention relates to an oil pressure control apparatus, and more particularly to an oil pressure control apparatus for controlling a servo mechanism.
2. Description of the Prior Art
A conventional oil pressure control apparatus 10 (FIG. 1) is utilized as one part of an oil pressure apparatus for controlling an oil pressure servo of a clutch portion 11 of vehicles. A mechanical portion of the apparatus includes the clutch portion 11 which is interposed between an engine 12 and a transmission 13 and transmits the power of the engine 12 to the transmission 13. The clutch portion 11 is of the type connected or disconnected (i.e., in torque transmitting or non-torque transmitting modes) by the application of oil pressure to the oil pressure servo (not shown) of the clutch portion 11.
An oil pressure apparatus portion of the oil pressure control apparatus 10 includes an oil pressure pump 14, a pressure regulating valve 15 and a drain valve 16. The pressure regulating valve 15 operates so as to apply the pressure change in a back pressure chamber 17 on a spool 18 via pressure regulating means 19. The valve 15 also regulates the oil pressure of the inlet port 20 at the spool 18 and delivers the regulated oil pressure to an outlet port 21 for delivery through an outlet line 26 to the clutch portion 11. The surplus pressurized oil is discharged through a drain port 22. The drain valve 16 is of a type in which a valve 23 is opened or closed by actuation of an electromagnetic coil 24.
The oil pressurized in the oil pressure pump 14 is supplied to the inlet port 20 of the pressure regulating valve 15 through a supply line 25. The pressurized oil whose pressure is regulated by the pressure regulating valve 15 is supplied to the oil pressure servo of the clutch portion 11 through the outlet line 26.
The pressurized oil for controlling the outlet pressure of the pressure regulating valve 15 is supplied to the back pressure chamber 17 of the pressure regulating valve 15 through a control line 27 branched from the supply line 25. A restricted orifice 28 is disposed in the control line 27 at the upstream side of the back pressure chamber 17 for limiting the supply of the pressurized oil to the chamber 17. The drain valve 16 is located at the downstream side of the back pressure chamber 17.
The operation according to the oil pressure control apparatus 10 of FIG. 1 is as follows:
When the current flows to the electromagnetic coil 24 of the drain valve 16, the valve 23 is opened and the pressurized oil within the control line 27 is discharged. The quantity of the oil supplied to the back pressure chamber 17 of the pressure regulating valve 15 is limited by the orifice 28, so that the pressure in the back pressure chamber 17 is decreased. The decreased pressure causes movement (rightward in FIG. 1) of the spool 18 such that the drain port 22 is opened and at the same time the opening of the inlet port 20 is limited. The pressurized oil at the outlet port 21 is discharged from the drain port 22 and hence the pressure on the oil pressure servo of the clutch portion 11 is decreased. When the spool 18 is completely moved rightwardly, the inlet port 20 is fully closed and the pressure on the oil pressure servo becomes zero and the clutch is disconnected.
Conversely, when the current flowing to the electromagnetic coil 24 is blocked, the valve 23 of the drain valve 16 is closed and the pressure in the back pressure chamber 17 of the pressure regulating valve 15 is increased and the spool 18 is moved in an opposite direction (i.e., to the left in FIG. 1). Accordingly, the spool 18 enlarges the opening of the inlet port 20 and simultaneously increases the oil pressure at the outlet port 21 by limiting the opening of the drain port 22, so that the pressure on the oil pressure servo of the clutch portion 11 is increased and the clutch is connected. The current flowing to the electromagnetic coil 24 is normally linearly changed.
The orifice 28 disposed within the control line 27 limits the quantity of the oil flowing to the back pressure chamber 17 and the back pressure is regulated by the drain valve 16 in the conventional oil pressure control apparatus. However, in the construction according to the conventional apparatus, when the valve 23 of the drain valve 16 is opened, the pressurized oil of the supplying line 25 is endlessly discharged via the orifice 28. Accordingly, the pressure loss becomes very large. An oil pressure pump having a large capacity is conventionally used for the oil pressure circuit in order to minimize the pressure loss. However, the use of such a large capacity pump is naturally uneconomical. As an alternative, the diameter of the orifice 28 may be decreased. In this case when the control signal for increasing the back pressure in the chamber 17 is sensed and the electromagnetic coil 24 is activated to close the valve 27, the response time is too long.